INRAE ​​researcher based at the University of Bordeaux, ecophysiologist Sylvain Delzon took samples with his team from maritime pines in the areas affected by the megafires this summer in La Teste-de-Buch, in the Bassin Arcachon, and in the Landiras and Hostens sector, south of Bordeaux.

The aim was in particular to provide the timber industry with immediate "decision support": cut or keep?

"Where there was a ground or basement fire, it was found that there was no cellular vitality at all at the base of the tree. Even though it was still completely green, it was going to die within 12 months. We recommended cutting," he explains.

For pines preserved at the base but whose green part (crown) has burned, survival is possible with "less than 70% damage to the crown".

Beyond that, the vitality "drops enormously", remarks Sylvain Delzon.

In production forests such as Landiras, all "stands" less than 10 years old have been destroyed but for very old trees (25 to 45 years old) the "survival rate" is "estimated at perhaps 25%" : "The bigger the trees, the more they were saved".

"Quality" wood

But "in La Teste-de-Buch and Hostens, the flames rose much higher. Even stands of 30 m were completely burnt. Probably because there was more undergrowth: this dry matter created very powerful fires."

The wood of the large trees affected, however, remained "of quality" for the exploitation, "even when the tree had burned to 100%", assures Sylvain Delzon.

According to him, "the operators have been able to sell as standard wood" the cuts made since October in the old stands of Landiras.

The surviving trees, on the other hand, seem more vulnerable to drought and "a little less productive" because the fires have damaged their vessels responsible for transporting water: "They will therefore have more difficulty growing".

The stricken pine forest in Landiras, Gironde, July 29, 2022 © Thibaud MORITZ / AFP/Archives

The problem remains moderate, however, because the thousands of hectares covered by the fire will "almost all be cut within four years".

At La Teste-de-Buch, in the part of the forest managed by the State, forestry machines began to extract the wood in September (the equivalent of ten years of cuts in normal times) before the arrival of insects, such as bark beetles, finish killing the most weakened trees.

Increase "genetic diversity"

Not far away, in the "user" part, private and not dedicated to production, the old rules of forest management complicate any decision-making between authorities, owners and users, but an auction (in batches) of dead trees has took place in December and the recovery of the wood will begin shortly.

Among the five campsites destroyed by fire, Les Flots Bleus, made famous by the "Camping" films, are in the midst of work to welcome the first holidaymakers in early April, with fewer pitches and an approach presented as more eco-responsible.

The "Les flots bleus" campsite damaged by forest fires on July 19, 2022 in Pyla-sur-Mer, Gironde © PHILIPPE LOPEZ / AFP/Archives

While the "States General of the Landes de Gascogne forest" have begun, Sylvain Delzon is also looking into the regeneration of the affected massifs.

In Landiras, the private owners are going to replant maritime pine but he recommends that they increase the "genetic diversity" of the plots by planting several types of this species.

This will promote resilience to climate change.

The researcher also advises "to leave as much hardwood as possible, as long as it does not interfere with the operation", in order to improve resistance to pests (insects, fungi).

In the "user" forest of La Teste, he recommends relying as much as possible on "natural regeneration".

Sylvain Delzon, however, defends a "certain clearing".

"Not systematic", to optimize biodiversity.

But "intelligent", to allow firefighters access and prevent undergrowth from forming continuously over hundreds of hectares.

He thus proposes to alternate between “cleaned and uncleaned plots”.

© 2023 AFP